1. Field of the Invention
The present invention relates to a crystal growth method, a crystal growth apparatus, a group-III nitride crystal, and a group-III nitride semiconductor device. In particular, the present invention relates to a crystal growth method and a crystal growth apparatus for a group-III nitride crystal, the group-III nitride crystal, and a group-III nitride semiconductor device-employing the group-III nitride crystal applicable to a blue light source for an optical disk drive, for example.
2. Description of the Related Art
Now, a InGaAlN-family (group-III nitride) device used as violet through blue through green light sources is produced by a crystal growth process employing an MO-CVD method (organic metal chemical vapor phase growth method), an MBE method (molecular beam crystal growth method), etc. on a sapphire or SiC substrate in most cases. In using sapphire or SiC as a substrate, crystal defect caused due to a large expansivity difference and/or lattice constant difference from a group-III nitride may occur frequently. By this reason, there is a problem that the device characteristic may become worth, it may be difficult to lengthen the life of the light-emission device, or the electric power consumption may become larger.
Furthermore, since a sapphire substrate has an insulating property, drawing of an electrode from the substrate like in another conventional light-emission device is impossible, and therefore, drawing the electrode from the nitride semiconductor surface on which crystal was grown is needed. Consequently, the device area may have to be enlarged, and, thereby, the costs may increase. Moreover, chip separation by cleavage is difficult for a group-III nitride semiconductor device produced on a sapphire substrate, and it is not easy to obtain a resonator end surface needed for a laser diode (LD) by cleavage, either. By this reason, a resonator end surface formation according to dry etching, or, after grinding a sapphire substrate to the thickness of 100 micrometers or less, a resonator end surface formation in a way near cleavage should be performed. Also in such a case, it is impossible to perform formation of a resonator end surface and chip separation easily by a single process like for another conventional LD, and, also, complication in process, and, thereby, cost increase may occur.
In order to solve these problems, it has been proposed to reduce the crystal defects by employing a selective lateral growth method and/or another technique for forming a group-III nitride semiconductor film on a sapphire substrate.
For example, a document xe2x80x98Japanese Journal of Applied Physics, Vol. 36 (1967), Part 2, No. 12A, pages L1568-1571xe2x80x99 (referred to as a first prior art, hereinafter) discloses a laser diode (LD) shown in FIG. 1. This configuration is produced as follows: After growing up a GaN low-temperature buffer layer 2 and a GaN layer 3, one by one, on a sapphire substrate 1 by an MO-VPE (organometallic vapor phase epitaxy) apparatus, an SiO2 mask 4 for selective growth is formed. This SiO2 mask 4 is formed through photo lithography and etching process, after depositing a SiO2 film by another CVD (chemistry vapor phase deposition) apparatus. Next, on this SiO2 mask 4, again, a GaN film 3xe2x80x2 is grown up to a thickness of 20 micrometers by the MO-VPE apparatus, and, thereby, GaN grows laterally selectively, and, as a result, the crystal defects are reduced as compared with the case where the selective lateral growth is not performed. Furthermore, prolonging of the crystal defect toward an activity layer 6 is prevented by provision of a modulation doped strained-layer superlattice layer (MD-SLS) 5 formed thereon. Consequently, as compared with the case where the selective lateral growth and modulation doped strained-layer superlattice layer are not used, it becomes possible to lengthen the device life.
In the case of this first prior art, although it becomes possible to reduce the crystal defects as compared with the case where the selective lateral growth of a GaN film is not carried out on a sapphire substrate, the above-mentioned problems concerning the insulating property and cleavage by using a sapphire substrate still remain. Furthermore, as the SiO2 mask formation process is added, the crystal growth by the MO-VPE apparatus is needed twice, and, thereby, a problem that a process is complicated newly arises.
As another method, for example, a document xe2x80x98Applied Physics Letters, Vol. 73, No. 6, pages 832-834 (1998)xe2x80x99 (referred to as a second prior art, hereinafter) discloses application of a GaN thick film substrate. By this second prior art, a GaN substrate is produced, by growing up a 200-micrometer GaN thick film by an H-VPE (hydride vapor phase growth) apparatus after 20-micrometer selective lateral growth according to the above-mentioned first prior art, and, then, grinding the GaN substrate thus having grown to be the thick film from the side of the sapphire substrate so that it may have the thickness of 150 micrometers. Then, the MO-VPE apparatus is used on this GaN substrate, crystal growth processes required for a LD device are performed, one by one, and, thus, the LD device is produced. Consequently, it becomes possible to solve the above-mentioned problems concerning the insulating property and cleavage by using the sapphire substrate in addition to solving the problem concerning the crystal defects.
A similar method is disclosed by Japanese Laid-Open Patent Application No. 11-4048. FIG. 7 shows a typical figure thereof.
However, further, the process is more complicated in the second prior art, and, requires the higher costs, in comparison to the first prior art. Moreover, in growing up the no less than 200 micrometer GaN thick film by the method of the second prior art, a stress occurring due to a lattice constant difference and a expansivity difference from the sapphire of the substrate becomes large, and a problem that the curvature and the crack of the substrate arise may newly occur. Moreover, even by performing such a complicated process, the crystal defective density can be reduced to only on the order of 106/cm2. Thus, it is not possible to obtain a practical semiconductor device.
In order to avoid this problem, setting to 1 mm or more thickness of an original substrate (sapphire and spinel are the most desirable materials as the substrate) from which a thick film grows is proposed by Japanese Laid-Open Patent Application No. 10-256662. According thereto, no curvature nor crack arise in the substrate even when the GaN film grows in 200 micrometers of thickness by applying this substrate having the thickness of 1 mm or more. However, a substrate thick in this way has a high cost of the substrate itself, and it is necessary to spend much time on polish thereof, and leads to the cost rise of the polish process. That is, as compared with the case where a thin substrate is used, the cost becomes higher by using the thick substrate. Moreover, although no curvature nor crack arise in the substrate after growing up the thick GaN film in using the thick substrate, curvature and/or crack may occur as stress relief occurs during the process of polish. By this reason, even when the thick substrate is used, the GaN substrate having a high crystal quality and having such a large area that it can be practically used for an ordinary semiconductor device manufacturing process cannot be easily produced.
A document xe2x80x98Journal of Crystal Growth, Vol. 189/190, pages. 153-158 (1998)xe2x80x99 (referred to as a third prior art, hereinafter) discloses that a bulk crystal of GaN is grown up, and it is used as a homoepitaxial substrate. According to this technique, under the high temperature in the range between 1400 and 1700xc2x0 C., and under the very high nitrogen pressure of 10 kilobars, crystal growth of the GaN is performed from a Ga liquid. In this case, it becomes possible to grow up a group-III nitride semiconductor film required for a device by using this GaN substrate. Therefore, it is possible to provide the GaN substrate without needing the process complicate like in the above-described first and second prior arts.
However, by this third prior art, crystal growth in high temperature and high pressure is needed, and, thus, there is a problem that a reaction vessel which can resist these conditions should be very expensive. In addition, even when such a growth method is employed, the size of the crystal obtained has the problem of being too small, i.e., at most on the order of 1 cm, and, thus, it is too small to put it in practical use of semiconductor device manufacture.
The GaN crystal growth method using Na which is an alkaline metal as a flux is proposed by a document xe2x80x98Chemistry of Materials, Vol. 9 (1977), pages 413-416xe2x80x99 (referred to as a fourth prior art, hereinafter) as a technique of solving the problem of GaN crystal growth in the above-mentioned high temperature and high pressure. According to this technique, sealing sodium azide (NaN3) and Ga metal used as a flux and a material into a reaction vessel made from stainless steel (vessel inner dimension: diameter=7.5 mm and length=100 mm) in nitrogen atmosphere, and the reaction vessel is maintained in the temperature in the range between 600 and 800xc2x0 C. for 24 to 100 hours to grow up a GaN crystal. In the case of this fourth prior art, crystal growth at the comparatively low temperature in the range between 600 and 800xc2x0 C. can be achieved, and, also, the require pressure inside the vessel should be only on the order of 100 kg/cm2, which is comparatively lower than the case of the third prior art. However, in this fourth prior art, the size of the crystal obtained is small as less than 1 mm which is too small to be put into practical use in semiconductor device manufacture, like in the case of the third prior art.
Therefore, the applicant of the present application has proposed a method of enlarging a group-III nitride crystal. However, in the method, nucleus generation initiates of the crystal growth is natural nucleus generation, and, thus, a large number of nucleus are undesirably generated. In order to control this nucleus generation, the applicant has proposed to utilize a seed crystal in the U.S. patent application Ser. No. 09/590,063, filed on Jun. 8, 2000, by Seiji Sarayama et al. (the entire contents of which are hereby incorporated by reference). However, there is a problem that a required crystal growth apparatus becomes complicated. Therefore, it has been demanded to realize a method for effectively controlling nucleus generation, while achieving a simple apparatus configuration of a conventional flux method, in order to solve this problem.
Further, Japanese Laid-Open Patent Application No. 2000-327495 discloses a fifth prior art combining the above-mentioned fourth prior art and an epitaxial method utilizing a substrate. In this method, a substrate on which GaN or AlN is grown previously is used, and, thereon, a GaN film according to the fourth prior art is grown. However, in this method, as it is basically the epitaxial method, the problem of crystal defects occurring in the above-mentioned first and second prior art cannot be solved. Further, as the GaN film or AlN film should be grown on the substrate previously, the process becomes complicated, and, thereby, the costs increase.
Furthermore, recently, Japanese Laid-Open Patent Applications Nos. 2000-12900 and 2000-22212 disclose a sixth prior art in which a GaAs substrate is used and a GaN thick-film substrate is produced. In this method, a GaN film having a thickens in a range between 70 xcexcm and 1 mm is selectively grown on a GaAs substrate by using an SiO2 film or SiN film as a mask as in the above-mentioned first prior art, as shown in FIGS. 3A through 3C. The crystal growth there is performed by the H-VPE apparatus. Then, the GaAs substrate is etched and thus removed by using aqua regia. Thus, the GaN self-standing substrate is produced, as shown in FIG. 3D. By using this GaN-self standing substrate, a GaN crystal having a thickness of several tens of millimeters is grown by vapor phase epitaxy by the H-VPE apparatus again, as shown in FIG. 4A. Then, this GaN crystal of several tens millimeters is cut into wafer shapes by a slicer, as shown in FIG. 4B. Thus, GaN wafers are produced, as shown in FIG. 4C.
According to this sixth prior art, the GaN self-standing substrate can be obtained, and, also, the GaN crystal having the thickness of several tens of millimeters can be obtained. However, this method has the following problems:
{circle around (1)} As the SiN film or SiO2 film is used as a mask for selective growth, the manufacturing process becomes complicated, and, thus, the costs increase;
{circle around (2)} When the GaN crystal having the thickness of several tens millimeters is grown by the H-VPE apparatus, GaN crystals (in monocrystal or polycrystal) or amorphous GaN having a similar thickness adhere to the inner wall of the reaction vessel. Accordingly, the productivity is degraded thereby.
{circle around (3)} As the GaAs substrate is etched and removed every time of the crystal growth as a sacrifice substrate, the costs increase thereby.
{circle around (4)} With regard to the crystal quality, problems of lattice mismatch due to crystal growth on a different-substance substrate, and a high defect density due to difference in expansivity remain.
An object of the present invention is to achieve a group III nitride crystal having a sufficient size such that a semiconductor device, such as a high-efficient light emitting diode or LD can be produced therefrom, without complicating the process which is the problem in the above-mentioned first or the second prior art, without using an expensive reaction vessel which is the problem in the third prior art, and without provision of insufficient size of the crystal which is the problem in the third and fourth prior arts, and, also, solving the above-mentioned problems in the fifth and sixth prior arts, and a crystal growth method and a crystal growth apparatus by which such a group-III nitride crystal can be manufactured, and a high-performance group-III nitride semiconductor device.
A crystal growth method according to the present invention, includes the steps of:
a) providing a nitrogen material into a reaction vessel in which a mixed molten liquid comprising an alkaline metal and a group-III metal; and
b) growing a crystal of a group-III nitride using the mixed molten liquid and the nitrogen material provided in the step a) in the reaction vessel,
wherein a provision is made such as to prevent a vapor of the alkaline metal from dispersing out of the reaction vessel.
Thereby, when growing up the group-III nitride crystal in the reaction vessel especially using the alkaline metal and the mixed molten liquid which contains group-III metal at least and the nitrogen material brought from the outside of the reaction vessel, the alkaline metal vapor is prevented from dispersing out of the reaction vessel. Thereby, evaporation of the alkaline metal out of the reaction vessel and condensation thereof can be prevented and it becomes possible to avoid obstruction against supply of the nitrogen material, and thus change of material composition. Consequently, the crystal growth can be well controlled, and a satisfactory group-III nitride crystal can be grown up stably.
A crystal growth method according to another aspect of the present invention includes the steps of:
a) providing a nitrogen material into a reaction vessel in which a mixed molten liquid comprising an alkaline metal and a group-III metal; and
b) growing a crystal of a group-III nitride using the mixed molten liquid and the nitrogen material provided in the step a) in the reaction vessel,
wherein a provision is made such as to prevent a vapor of the alkaline metal from blocking a zone through which the nitrogen material is supplied from the outside of the reaction vessel.
Thereby, the nitrogen material brought in from the outside of the reaction vessel can be prevented from being blocked by the condensed alkaline metal.
Consequently, the crystal growth can be well controlled, and, a satisfactory group-III nitride crystal can be grown up stably.
For this purpose, the temperature in the reaction vessel above the surface of the mixed molten liquid may be preferably controlled so as to prevent the vapor of the alkaline metal from condensing.
The temperature of the above-mentioned zone may preferably be controlled for the same purpose.
Further, another reaction vessel may be provided outside of the reaction vessel;
the nitrogen material may be brought into the reaction vessel through this outer reaction vessel; and
a provision may preferably be made such as to allow the nitrogen material to be brought into the originally provided inner reaction vessel from the outer reaction vessel, and, also, to prevent the vapor of the alkaline metal from dispersing out of the inner reaction vessel, for the above-mentioned object.
The nitrogen material may be preferably supplied horizontally or from a direction below the horizontal direction.
Thereby, condensation of the alkaline metal vapor in the zone through which the nitrogen material is supplied can be prevented.
A crystal growth apparatus according to the present invention includes:
a reaction vessel holding a mixed molten liquid comprising an alkaline metal and a group-III metal;
a first heating device heating the mixed molten liquid so as to enable crystal growth therein; and
a second heating device heating above the surface of the mixed molten liquid so as to prevent the vapor of the alkaline metal above the surface of the mixed molten liquid from condensing.
A crystal growth apparatus according to another aspect of the present invention includes:
a reaction vessel holding a mixed molten liquid comprising an alkaline metal and a group-III metal; and
a heating device heating a zone through which a nitrogen material is supplied externally into the reaction vessel.
Thereby, a complicated process described above for the first or second prior art is not needed, but it becomes possible to obtain a high-quality group-III nitride crystal at low cost. Furthermore, the required growth temperature is as low as less than 100xc2x0 C., and, also, the required growth pressure is as low as less than 100 kg/cm2, for the crystal growth of the group-III nitride. Accordingly, it is not necessary to use an expensive reaction vessel which can resist a super-high pressure and a super-high temperature as in the above-mentioned third prior art. Consequently, it becomes possible at low cost to obtain a group-III nitride crystal. Moreover, since it is low temperature and low pressure needed for the crystal growth, it becomes possible by using a seed crystal as a nucleus to enlarge the size of the group-III nitride crystal by carrying out crystal growth.
A crystal growth method according to another aspect of the present invention includes the steps of:
a) carrying out crystal growth in a reaction vessel of a group-III nitride comprising a group-III metal and a nitrogen from an alkaline metal, a substance comprising the group-III metal, and a substance comprising the nitrogen; and
b) maintaining a growth condition for a crystal the group-III nitride at a condition at which the crystal growth starts; then,
c) maintaining the growth condition at a condition at which the crystal growth stops; and, then,
d) again setting the condition at which the crystal growth starts.
Thus, by setting the crystal growth condition enabling the crystal growth and then setting the other crystal growth condition not enabling the crystal growth, a crystal nucleus can be grown selectively. That is, by setting again the crystal growth condition enabling the crystal growth, the crystal growth progresses further from this crystal nucleus. By repeating such a control of the crystal growth condition as that the crystal growable condition is entered and exited from, it is possible to control generation of crystal nucleus, in comparison to a case where no such a control is performed. Thus, it becomes possible to grow the group-III nitride crystal to have a large size effectively, and thus to effectively utilize the materials therefor. As a result, it is possible to obtain a large-sized group-III nitride crystal at low cost.
Further, in comparison to a seed-crystal method in the related art in which a position of a crystal nucleus supplied externally as a seed crystal is controlled, the apparatus is not needed to be so complicated, and, thus, the total cost can be reduced, according to the present invention.
Specifically, the step b) may maintain the temperature of a zone in which a crystal of the group-III nitride grows at a temperature at which the crystal growth starts;
the step c) may lower the temperature of the zone to a temperature such that no alloy is formed between the group-III metal and another metal, and maintaining this temperature; and
the step d) may increase the temperature to the temperature at which the crystal growth starts again.
The increase and decrease of the temperature may be preferably performed several times.
The substance comprising the nitrogen may be of a gas, and the gas may be supplied into the reaction vessel continuously at a predetermined pressure. Thereby, it is possible to control the crystal growth reaction only by control of the temperature. As a result, it is possible to control a change in growth parameter in the crystal growth, and, also, by continuously supplying the nitrogen material, a high-quality group-III nitride crystal can be grown with little nitrogen loss.
The substance comprising the group-III metal may preferably be additionally provided at a time of the temperature is lowered.
Thereby, it is possible to avoid a situation of unexpected interruption of the crystal growth occurring due to exhaustion of the group-III material. Furthermore, it is possible to effectively prevent change of the ratio in amount among the group-III material and group-V material, and the alkaline metal used as the flux. As a result, it is possible to achieve stable crystal growth wherein the crystal quality is fixed stably, and, thus, it is possible to grow up a high-quality group-III nitride crystal.
Furthermore, as the timing of the additional supply of the group-III material is in an interval in which the crystal growth is terminated, it is possible to effectively control change in grow parameter such as temperature change, material amount ratio change and so forth which may otherwise adversely affect the proper crystal growth. Also by this point, the crystal growth for a high-quality group-III nitride crystal can be more positively achieved.
The above-mentioned step b) may instead maintain an effective pressure of the substance comprising the nitride in a form a gas in a zone in which a crystal of the group-III nitride grows at a pressure at which the crystal growth starts;
the step c) may lower the effective pressure of the nitrogen gas in the zone to a pressure such that the crystal growth stops, and maintaining this pressure; and
the step d) may increase the effective pressure of the nitrogen gas to the pressure at which the crystal growth starts again.
Further, a crystal growth apparatus which carries out crystal growth of the group-III nitride crystal which has the features described above can be realized at low cost in addition to the above-mentioned effects.
Furthermore, by carrying out the crystal growth according to any one of the above-described methods and/or the above-mentioned apparatuses, it becomes possible to realize a large-sized group-III nitride crystal by which a semiconductor device may be produced in a practical manner at low cost.
Furthermore, by producing the group-III nitride semiconductor device using the group-III nitride crystal mentioned above, a highly efficient device is realizable at low cost. This group-III nitride crystal is a high-quality crystal having few crystal defects, as mentioned above. Thus, a highly efficient device is realizable by device production from thin film growth using this group-III nitride crystal, or using it as a substrate of the device. That is, a high output which has not been realized conventionally can be provided by the device and a long life of the device is achieved in a case of production of a semiconductor laser or a light emitting diode therefrom. In a case of production of an electronic device therefrom, low power consumption, low noise, high-speed operation, and high temperature operation are achievable therefrom. In a case of light receiving device, low noise and a long life can be obtained therefrom.
A crystal growth method according to another aspect of the present invention includes the steps of:
a) forming a mixed molten liquid comprising an alkaline metal and a substance comprising a group-III metal in a liquid holding vessel;
b) growing in the liquid holding vessel a crystal of a group-III nitride comprising the group-III metal and nitride from the mixed molten liquid and a substance comprising the nitride;
c) creating a local concentration distribution of dissolved nitrogen in the mixed molten liquid in the liquid holding vessel during the step b).
Thereby, without making the process complicated as in the first and second prior arts described above, since the local concentration distribution of the dissolved nitrogen is produced in the mixed molten liquid, it becomes possible to avoid use of an expensive reaction vessel as in the third prior art, and the size of the produced crystal can be enlarged in contrast to the third and fourth prior art. Thus, the group-III nitride crystal of a practical size for producing semiconductor devices, such as a highly efficient light emitting diode and LD, can be grown up.
Furthermore, the necessary growth temperature is as low as 1000 degrees C. or less, and, also, the necessary growth pressure is as low as approximately 100 or less atm. Thereby, it is not necessary to use an expensive reaction vessel which can resist a super-high pressure and a super-high temperature as in the third prior art. Consequently, it becomes possible to realize the device using the group-III nitride crystal at low cost.
Furthermore, by producing the local concentration (uneven) distribution of the dissolved nitrogen in the mixed molten liquid, it becomes possible to limit a location of occurrence of nucleus generation of the group-III nitride crystal to a specific part of the mixed molten liquid, and the group-III nitride crystal having a large size can thus be grown up.
The liquid holding vessel may have an inner shape such as to produce the local concentration distribution of the dissolved nitrogen in the mixed molten liquid.
The inner shape of the liquid holding vessel may be such that the cross sectional area becomes smaller downward.
The inner shape of the liquid holding vessel may instead be such that the cross sectional area is reduced partially (at a specific height).
The inner shape of the liquid holding vessel may future instead be such that the cross sectional area becomes smaller downward first, and, then, the cross sectional area is uniform downward from the mid level (height).
The inner shape of the liquid holding vessel may further instead be such that the cross sectional area becomes smaller downward first, and, then, the cross sectional area becomes larger downward from the mid level.
A crystal growth apparatus according to another aspect of the present invention includes:
a liquid holding vessel in which a mixed molten liquid comprising an alkaline metal and a substance comprising a group-III metal is formed; and
a unit growing in the liquid holding vessel a crystal of a group-III nitride comprising the group-III metal and nitride from the mixed molten liquid and a substance comprising the nitride, and,
wherein the liquid holding vessel has an inner shape such as to produce a local concentration distribution of dissolved nitrogen in the mixed molten liquid (as mentioned above in the crystal growth methods).
The above-mentioned unit may include a heating device heating the temperature inside the liquid holding vessel so as to enable the crystal growth therein.
The unit may include a plurality of heating devices for creating a predetermined temperature difference between an upper part and a lower part of the liquid holding vessel independently.
Thus, since the cross sectional area of the vessel becomes smaller downward, and, then, it is uniform from the mid level, or it becomes larger from the mid level, the mixed molten liquid may be held to this zone. Consequently, the group-III metal can be continuously supplied therefrom to a specific zone in which the crystal nucleus is generated, and, thereby, it becomes possible to grow up a large-sized group-III nitride crystal.
Moreover, the group-III nitride crystal thus produced has a high quality (few crystal defects), and also, has a large size such as to be practically utilized for producing a semiconductor device, and such a group-III nitride crystal can be produced at low cost.
Moreover, since it is the semiconductor device produced using the group-III nitride crystal according to the present invention described above, a highly efficient group-III nitride semiconductor device can be offered at low cost.
Furthermore, by producing the group-III nitride semiconductor device using the group-III nitride crystal mentioned above, a highly efficient device is realizable at low cost. As this group-III nitride crystal is a high-quality crystal having few crystal defects, as mentioned above, a highly efficient device is realizable by device production from thin film growth using this group-III nitride crystal, or using it as a substrate of the device. That is, a high output which has not been realized conventionally can be provided and a long life is provided in a case of production of a semiconductor laser or a light emitting diode. In a case of production of an electronic device, low power consumption, low noise, high-speed operation, and high temperature operation are achievable. In a case of light receiving device, low noise and a long life can be obtained.
Moreover, according to the present invention, the semiconductor device may be a light-emission device which emits light of the wavelength shorter than 400 nm, and can emit light at high efficiency also in this wavelength region. That is, since the semiconductor device thus obtained has few crystal defects and few impurities consequently, it becomes possible to realize the efficient light-emission characteristic wherein light emission from a deep level is well controlled.